1. Field of the Invention
The present invention relates to a carbazole derivative. In addition, the present invention relates to a material for a light-emitting element, a light-emitting element, an electronic device, and a lighting device each of which uses the carbazole derivative.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements utilizing, electroluminescence. In a basic structure of such a light-emitting element, a layer containing a light-emitting substance is interposed between a pair of electrodes. By voltage application to this element, light emission can be obtained from the light-emitting substance.
Since such a light-emitting element is a self-light-emitting type, it has advantages over a liquid crystal display in that visibility of pixels is high, a backlight is not required, and so on. Accordingly, such a light-emitting element is considered suitable as a flat panel display element. In addition, other advantages of such a light-emitting element are that it can be manufactured to be thin and lightweight and the response speed is very high.
Furthermore, since such a light-emitting element can be formed into a film form, planar light emission can be easily obtained by forming a large-area element. This feature cannot be easily obtained by point light sources typified by an incandescent lamp and an LED or linear light sources typified by a fluorescent lamp. Accordingly, the light-emitting element is extremely effective for use as a surface light source applicable to lighting and the like.
Light-emitting elements utilizing electroluminescence are broadly classified according to whether they use an organic compound or an inorganic compound as a light-emitting substance. When an organic compound is used as a light-emitting substance, by voltage application to a light-emitting element, electrons and holes are injected into a layer including the light-emitting organic compound from a pair of electrodes, whereby current flows. The carriers (electrons and holes) are recombined, and thus, the light-emitting organic compound is excited. The light-emitting organic compound returns to a ground state from the excited state, thereby emitting light.
Because of such a mechanism, the light-emitting element is referred to as a current-excitation light-emitting element. Note that the excited state of an organic compound can be either a singlet excited state or a triplet excited state, and light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence.
In improving element characteristics of such a light-emitting element, there are a lot of problems which depend on a substance, and in order to solve the problems, improvement of an element structure, development of a substance, and the like have been carried out (for example, see Non-Patent Document 1).